1. Field of the Invention
The present invention relates to a device and method for reshaping a corneal surface of an eye for refractive correction by laser ablation. More particularly, it relates to an improved device and method for corneal reshaping by laser ablation for correcting a simple astigmatism condition.
2. Description of Related Art
While many people have perfect eyesight, many others do not. One reason for imperfect eyesight is caused by refractive disorders of the eye. Astigmatism is one possible refractive disorder of the eye caused by an out-of-round cornea. Astigmatism results in light from a point source being focused in an image line rather than a point.
One method of treating simple astigmatism is through the use of conventional laser refractive surgery. Laser refractive surgery utilizes a laser to ablate and thus remove layers of corneal tissue in a predetermined pattern to correct the refractive disorder. Typically, such laser refractive surgery ablates many layers of corneal tissue with the cumulative effect attempting to form, e.g., a narrow, cylinder-shaped portion removed from the cornea. However, conventional apparatus and method only approximate removal of a cylinder-like portion, and a smooth surface on the remaining corneal tissue is often difficult to achieve, leading to problems of clarity of eye sight.
It is known to ablate or remove a narrow, cylinder-shaped portion from the corneal surface of the eye to correct this particular eye disorder, as illustrated in FIGS. 2a-2c. FIG. 2a depicts the ideal surface of the cornea after a narrow, cylinder-shaped portion is removed. FIGS. 2b and 2c show the ideal surface of the cornea in cross section along the x-axis and y-axis, respectively.
Ideally, this correction corrects astigmatism. Unfortunately, the real world is less than ideal. For instance, FIG. 2d depicts the actual surface of the cornea in cross section along the y-axis resulting from conventional laser refractive surgery methods and apparatus. The sharp, ideal corners 200 of the ablated area are actually somewhat rounded. Moreover, although substantially flat after initial ablation, cylindrical correction of simple astigmatism is known to induce hyperopia, another refractive disorder of the eye, along, e.g., the flat or y-axis, as illustrated in FIG. 2e. The induced hyperopia is caused by corneal re-growth or regression after the laser refractive surgery away from the corneal surface 202 initially after ablation.
Furthermore, certain conventional laser refractive surgeries utilizing a small, low-energy laser beam scan, each corneal tissue layer is ablated by a large number of equally spaced laser pulses with each laser pulse partially overlapping adjacent laser pulses. Unfortunately, in a small, low-energy laser is beam scan, the first laser beam scan and the last laser beam scan do not always fall on the pre-defined border of the ablation zone of a particular corneal tissue layer.
For example, FIG. 4 depicts a conventional method of scanning an ablating laser beam across a generally rectangular ablation zone. As shown, the ablation points may or may not be centered on a predetermined border 402. For instance, although the ablation points 404 may be centered on the starting border 406, the last ablation points in a series of scan lines may not be centered on the ending border 402.
Thus, by not knowing where the starting point and/or ending point will land, the eye surgeon has a difficult time achieving an accurate layer width and centration in scanning ablation.
Lastly, since the cylindrical ablation is usually narrow, the total depth of ablation is typically small, and therefore the number of corneal tissue layers to be ablated is small. The typical thickness of a layer of corneal tissue is approximately two (2) micrometers (xcexcm), and because the number of corneal tissue layers to be ablated is typically small, the effectiveness of the procedure is generally impacted. A small number of layers causes inaccuracy of total depth of ablation because each layer of the few layers represents a reasonably large percentage of the total depth to be ablated. For example, if three and one-half (3.5) layers at two (2) xcexcm each are required to be ablated to result in a total depth of seven (7) xcexcm, the eye surgeon must choose between an ablation of either three (3) layers or four (4) layers, since it is not feasible to ablate only half a corneal tissue layer. By ablating either three (3) layers or four (4) layers instead of the ideal 3.5 layers, significant error is caused in the ablation, i.e., approximately fourteen (14%) percent, and thus the refractive disorder may not be completely corrected.
Accordingly, there exists a need for an improved device and method for correcting simple astigmatism by laser ablation which obviates and overcomes many of the disadvantages and shortcomings experienced with the prior laser surgery devices. For instance, there is a need for an improved device and method for correcting simple astigmatism by laser ablation which does not induce hyperopia. Furthermore, there is a need to correct simple astigmatism by laser ablation more accurately along a predefined border of the ablation zone. Lastly, there is a need to improve the inherent error otherwise caused in the ablation of a small number of corneal tissue layers.
In accordance with the principles of one aspect of the present invention, astigmatism is corrected with apparatus comprising an ablating pulsed laser beam, and a scanner to scan the ablating pulsed laser beam in scan paths across an ablation zone of the eye. A rotator rotates an angle of the scan paths with respect to the ablation zone based on a step size between pulses of the ablating pulsed laser beam.
A method of scanning a laser beam across an ablation zone of an eye is also provided. According to the method, a total number of ablation layers to perform a particular refractive correction of the eye are determined. An approximate step size between each ablation point in scan lines across the ablation zone is determined. A rotation of the scan lines with respect to the ablation zone based on at least one of the total number of ablation layers and the approximate step size is determined, and the laser beam is scanned in a predetermined pattern based on the rotation of the scan lines.
In accordance with another aspect of the present invention, apparatus is provided for forming a cylindrical-shaped ablation in a cornea of an eye. The apparatus comprises an ablating pulsed laser beam, and a scanner to scan the ablating pulsed laser beam in a plurality of scan paths across an ablation zone of the eye to remove a plurality of layers of corneal tissue. The scanner progressively decreases the lengths from a first to last one of the plurality of layers.
In accordance with another method of the present invention, astigmatism is corrected in an eye by scanning an ablating laser beam in a predetermined pattern across an ablation zone of the eye to form a cylindrical-shaped ablation in a cornea of the eye. Moreover, a transition region is further ablated in each end of the cylindrical-shaped ablation.
In accordance with yet another aspect of the invention, astigmatism is corrected in an eye by identifying a center of a refractive correction in the eye, and a rectangular ablation zone is determined about the center. A maximum depth of ablation is determined, and a total number of ablation layers corresponding to said maximum depth is determined. A scanning pattern is determined for each of the ablation layers, with each of the scanning patterns including a step size between pulses of an ablating laser based on the desired total number of ablation layers. Each of the ablation layers is then ablated with the respectively determined scanning patterns.